Fermat's principle

by pi-r8
Tags: fermat, principle
 P: 141 I don't understand this idea. My terxtbook says that Fermat's principle is that light travels by the path that takes the least amount of time. Does that mean that light will go in crazy, curved paths if those are faster? How does it "know" which path will be the fastest? For example, let's say I'm shining a flashlight towards a block of some medium with a really high index of refraction, so light traveling in this medium goes really slowly. If light is going in a straight line from the flashlight to a point directly behind the medium, it's going to take a long time to get there. On the other hand, if it follows a curved path up and over the medium, then down behind it, the total time would be much less. Is that what happens?
 P: 246 Your understanding of his principle is flawed somewhat, read on Wikipedia or anywhere else to see why!
 PF Patron P: 8,963 This isn't one of my areas, so I'll just give a brief response. Light does not 'travel' through a refractive medium. When a photon enters the medium, it's absorbed by an atom. A new photon is then emitted, which usually matches the original. That new photon then gets absorbed by another atom, and so on and so on...
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Fermat's principle

Here is a good starting point:
http://www.eftaylor.com/leastaction.html
http://www.eftaylor.com/quantum.html
 P: 141 The wikipedia entry on Fermat's principle isn't particularly helpful... just the same old stuff about how you can use it to derive snell's law (which I had to do as a homework problem this year).
 P: 141 robphy- I know about the principle of least action. What I don't understand is the actual, physical meaning of Fermat's principle. Even if, as Danger said, photons are constantly absorbed and emitted, rather than just traveling straight through, I don't understand why light would take the shortest time path, rather than a straight path.
P: 246
 Quote by Wikipedia!! The modern, full version of Fermat's Principle states that the optical path length must be extremal, which means that it can be either minimal, maximal or a point of inflection (a saddle point). Minima occur most often, for instance the angle of refraction a wave takes when passing into a different medium or the path light has when reflected off of a planar mirror. Maxima occur in gravitational lensing. A point of inflection describes the path light takes when it is reflected off of an elliptical mirrored surface.
Hope that helps
 P: 141 um... not really. How does that explain my example of the flashlight and the block? In real life the light goes straight through the block... but the minimum path would be to go around the block, and the maximum path would be infinitely long. I dunno what the point of inflection path would be though... is that it?
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